because tanks are typically constructed of food-safe materials, you should be able to leave your liquids sitting in 99% of them without issue. I have left liquid in tanks for months, personally. glass, stainless steel, cotton & kanthal are all pretty inert, even when left sitting in
juice.
some tanks are made of food safe plastics and some liquids have been known to etch these plastics (cinnamon) in which case, if you pickup a plastic tank that's been sitting for a year and it's hazy or cracked, maybe dump the juice and move on.
also, if you let a relatively light juice sit for a few days (especially in a tank with a wick you've burnt in the past) you may see darker liquid leaching from the coil feed holes of your coil head. should be fine. just discoloration. (from being in close proximity to the heat source)